1. Field of the Invention
The present invention relates to a multi-user detection method for a receiver in a multi-carrier code division multiple access system and more specifically to a multi-user detection method with multiple access interference cancellation. The present invention concerns also a MC-CDMA receiver implementing such a multi-user detection method.
2. Description of the Background Art
Multi-Carrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
Unlike the DS-CDMA (Direct Sequence Code Division Multiple Access) method, in which the signal of each user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of the signature multiplying the signal of a different sub-carrier.
More precisely, FIG. 1 shows the structure of an MC-CDMA transmitter for a given user k. Let bk(i) be the ith symbol to be transmitted from the user k, where bk(i) belongs to the modulation alphabet. The symbol bk(i) is first of all multiplied at 110 by a spreading sequence or signature of the user, denoted ck(t), consisting of N “chips”, each “chip” being of duration Tc, the total duration of the spreading sequence corresponding to a symbol period T. The results of the multiplication of the symbol bk(i) by the different “chips” are converted by the serial to parallel converter 120 into a block of L symbols, where L is in general a multiple of N. It will be considered, for reasons of simplification of presentation, that L=N. The block of L symbols is then subjected to an inverse fast Fourier transformation (IFFT) in the module 130. In order to prevent intersymbol interference, a guard interval, with a length greater than the duration of the pulse-type response of the transmission channel, is added to the MC-CDMA symbol. This interval is obtained by appending a suffix (denoted Δ) identical to the start of the said symbol. Before being transmitted to the parallel to serial converter 140, the symbols thus obtained are amplified at 150 in order to be transmitted over the user channel. It can therefore be seen that the MC-CDMA method can be analysed into a spreading in the spectral domain (before IFFT) followed by an OFDM modulation.
In practice, the user k transmits his data in the form of frames of I symbols, each symbol bk(i) being spread by a real signature ck(t), typically a Walsh-Hadamard signature, with a duration equal to the symbol period T, such that ck(t)=0 if t ∉[0, T[. The signal modulated at time t=(i−1).T+(l−1).Tc can then be written, if the guard intervals between MC-CDMA symbols are omitted:
                                          S            k                    ⁡                      (            t            )                          =                              ∑                          i              =              1                        I                    ⁢                                          ⁢                                    ∑                              l                =                1                            L                        ⁢                                                  ⁢                                          ω                k                            ·                                                c                  k                                ⁡                                  (                                                            (                                              l                        -                        1                                            )                                        ·                                          T                      c                                                        )                                            ·                                                b                  k                                ⁡                                  (                                      i                    -                    1                                    )                                            ·                              exp                ⁡                                  (                                                            j                      ·                      2                                        ⁢                                                                  π                        ⁡                                                  (                                                      l                            -                            1                                                    )                                                                    /                      L                                                        )                                                                                        (        1        )            where ωk is the amplitude of the signal transmitted by the user k, assumed to be constant for a transmission unit.
An MC-CDMA receiver for a given user k has been illustrated schematically in FIG. 2. This receiver is known as single-user detection receiver (or SUD receiver) because the detection takes only into account the symbols transmitted to (or from) the user in question.
The demodulated received signal is sampled at the “chip” frequency. The signal thus obtained can be written:
                              R          ⁡                      (            t            )                          =                                            ∑                              k                =                1                            K                        ⁢                                                  ⁢                                          ∑                                  i                  =                  1                                I                            ⁢                                                          ⁢                                                ∑                                      l                    =                    1                                    L                                ⁢                                                                  ⁢                                                                            h                      kl                                        ⁡                                          (                                              i                        -                        1                                            )                                                        ·                                      ω                    k                                    ·                                      c                    kl                                    ·                                                            b                      k                                        ⁡                                          (                                              i                        -                        1                                            )                                                        ·                                      exp                    ⁡                                          (                                                                        j                          ·                          2                                                ⁢                                                                              π                            ⁡                                                          (                                                              l                                -                                1                                                            )                                                                                /                          L                                                                    )                                                                                                    +                      n            ⁡                          (              t              )                                                          (        2        )            where K is the number of users, ckl=ck((l−1).Tc), hkl(i) represents the response of the channel of the user k to the frequency of the subcarrier l of the MC-CDMA symbol transmitted at time i.T and where n(t) is the received noise.
If the downlink channel is considered, the transmission channels have identical characteristics and therefore hkl=hl.
The samples obtained by sampling at the “chip” frequency are put in parallel in a serial to parallel converter 210 and stripped from the suffix (Δ) before undergoing an FFT in the module 220.
In MC-CDMA, the presence of the guard period makes it possible to neglect the intersymbol interference. For a given subcarrier (hereinafter simply called carrier), the equalisation can therefore be performed by a single tap, i.e. by a multiplication by a complex coefficient. In the SUD receiver, the equalisation is performed carrier-by-carrier i.e. by applying one of the known equalisation methods: MRC (Maximum Ratio Combining), EGC (Equal Gain Combining), ZF (Zero Forcing) or MMSE (Minimum Mean Square Error) independently on each carrier. The equalising coefficients thus obtained are denoted qk,1, . . . , qk,L on FIG. 2.
The samples in the frequency domain, output from 220, are multiplied by the equalising coefficients and the signature of the user k in 2400, . . . , 240L-1 (for despreading) before being added in 250. The result is an estimation {circumflex over (b)}k(i) of the transmitted symbol bk(i).
The multiuser detection techniques are known notably in CDMA telecommunications systems. They have the common characteristic of taking account of the interference generated by the other users.
A multiuser detection or MUD technique for MC-CDMA was presented in the article by J-Y. Beaudais, J. F. Hélard and J. Citerne entitled “A novel linear MMSE detection technique for MC-CDMA” published in Electronics Letters, Vol. 36, No. 7, pages 665-666, 30 March 2000. The equalisation method proposed no longer operates carrier by carrier but MC-CDMA symbol by MC-CDMA symbol, taking account of all the carriers and all the signatures of the active users. For this reason it is called GMMSE (Global Minimum Mean Square Error) equalisation or, equivalently, M-MMSE (Matrix Minimum Mean Square Error) equalisation. Its purpose is to minimise the mean square error between the estimated symbols {circumflex over (b)}k(i) and the transmitted symbols bk(i).
An MC-CDMA receiver with GMMSE equalisation for a user k has been shown in FIG. 3. Its structure differs from that of FIG. 2 in that the equalisation is effected by means of a multiplication 330 by a matrix Q of the signals of the different carriers. The despread signal obtained at the output of the adder 350 gives the estimated symbol {circumflex over (b)}k(i).
Furthermore, structures of MUD MC-CDMA receivers using either a parallel interference cancellation (PIC) scheme or a serial interference cancellation scheme (SIC) have been proposed in the article by J-Y. Beaudais et al. Parallel interference cancellation consists in iteratively cancelling the multiple access interference (MAI) by subtracting for a given user the contributions due to the other users, these contributions being obtained, for a given step, by respreading the symbols estimated at the previous step and filtering them with filters modelling their respective transmission channels. In contrast, serial interference cancellation consists in successively eliminating the contributions of the users in a cascade of stages, each stage eliminating the contribution of a particular user, commencing by the contribution of highest power.
FIG. 4 illustrates the structure of a MC-CDMA receiver with parallel interference cancellation. For the sake of clarity, only the part related to a user k has been represented. In FIG. 4, r(i) denotes the output of the FFT, i.e. the vector of frequency components supplied by stage 320 in FIG. 3 at time i.T. These components are subjected to a first equalisation in 410. More precisely, 410 provides for an equalisation for the K-1 transmission channels associated with the users k′=1, . . . K, k′≠k. If we consider the downlink receiver, the transmission channels are identical, the equalisation 410 amounts to a simple multiplication by an L×L matrix and the output is a vector of dimension L. In the uplink receiver however, K-1 matrices are involved, each matrix corresponding to a transmission channel of a user. These K-1 matrices are represented by the tensor {tilde over (Q)}(1), where the index (1) stands for the first cancellation stage. After equalisation, the received MC-CDMA symbols are despread in 420 with the K-1 signatures of the users k′=1, . . . K, k′≠k. and detected in 430. This first estimation does not take into account the MAI. The detected symbols are then respread by said signatures in 440 and the MC-CDMA symbols thus obtained are filtered by K-1 filters modelling the transmission channels of the users k′. Each filter can be represented by an L×L diagonal matrix and the K-1 matrices are represented by the tensor {tilde over (H)}. Here again, if the downlink receiver is considered, the latter matrices are identical. In any case, the K-1 outputs of 450 give the respective contributions of the users k′ to the MAI. They are subtracted from r(i) (delayed so as to compensate for the processing time in 410-450) in 4601, . . . , 460K-1. The output of 460K-1 is a version of r(i) freed of a first estimation of MAI. The process can be repeated so as to refine the estimation of the MAI and jointly improve the detection of the symbols. The last stage is followed by a classical single-user equalisation in 470, despreading in 480 and symbol detection in 490 as in FIG. 2.
The proposed parallel and serial cancellation schemes in MC-CDMA are rather complex in particular for the uplink receiver since some processing stages are performed at the carrier level and therefore involve (K-1).L operations in parallel where K is the number of users and L is the number of carriers.